The present invention is related to control systems for steam turbines and, more particularly, to control systems for controlling torque of variable speed steam turbines.
Steam turbines are principally used as prime movers although there are some instances in which such turbines are applied as loads for other prime movers or occasionally as steam pressure reducers. When used as either a prime mover or a load, a shaft of the turbine is coupled through a clutch or other type connector to a rotating shaft of a driving or driven apparatus. If the torque reflected from the apparatus to the turbine exceeds the torque handling capacity of the connector, the connector may be damaged or destroyed. Alternately, the shaft of the turbine or the apparatus may be overstressed leading to deformation or breakage.
Prior art control systems for turbines have basically concentrated on a speed control, i.e., maintaining a constant shaft speed without regard for torque. Provided that the turbine and attendant connections are over-designed, such speed controls are satisfactory. However, over-designing the turbine and connections is not economical and cannot assure that a torque will not be experienced which exceeds even the over-designed characteristics. Furthermore, such speed controls tend to aggravate overload situations by attempting to generate additional torque in order to maintain a constant speed.